Centerless coaxial connector



April 23, 1963A M. A. MAURY ETAI. 3,087,129

cENTERLEss coAxIAL CONNECTOR Filed Feb. 25, 1960 INVENToRs Mar/'o A.Maury /a /0/ 2/ /9 Henry K. Om/ya ATTORNEYS Unite 3,087,129 CENTERLESSCOAXIAL CNNECTOR Mario A. Maury, Montclair, and Henry K. Omiya,Fullerton, Calif., assigncrs, by mesne assignments, to the United Statesof America as represented by the Secretary of the Navy Filed Feb. 25,1960, Ser. No. 11,1% 6 Claims. (Cl. 333-9) The present invention relatesto coaxial cables and more particularly to new and improved endterminations for such coaxial cables capable of forming wide bandcenterless coaxial connectors.

For a number of years in the field of microwave transmission, it hasbeen the general practice to employ coaxial cables for the conduction ofradio frequency energy. In missile work, as well as other physicallydemanding electronic applications, it is frequently necessary to coupleradio frequency energy from a first coaxial transmission line to asecond coaxial transmission line in the form of a quick disconnect. Thelatter expedient for coupling coaxial cables must possesscharacteristics of extreme ruggedness, if it is to withstand the veryhigh orders of vibration, temperature and shock commonly encountered inmissiles and related devices.

Before the introduction of the coupling device of the V presentinvention, the prior art coupling expedients made use of mechanicalcontact fingers for coupling the conductors of one coaxial cable tothose of an adjacent cable. However, such contact fingers have alwayshad the rather stringent requirement that the connectors be accurately`aligned if the contact fingers were -to be properly seated, as theymust for proper performance. This last requisite imposes an extremedesign problem since lmanufacturing tolerances for connectors utilizingsuch contact fingers must be kept within highly prohibitive figures,making the practicality of such a coupling expedient questionable. Itcan be readily visualized that the use of such contact fingers forcoupling microwave transmission lines under extreme environmentalconditions, where accurate alignment would be difficult, if notimpossible, presents ia serious problem. Those concerned with thedevelopment of coaxial connectors have long recognized the need for aquick disconnect device obviating the use of such ltroublesome contactfingers and the present invention clearly fulfills this need.

The general purpose of this invention is to provide a coaxial connectorwhich embraces substantially all of the advantages of similarly employedcoaxial connectors and possesses none of the aforedescribeddisadvantages. To attain this, the present invention contemplates aunique end termination for coaxial cables obviating the use of contactfingers by providing for a transition in mode of transmission at thepoint of coupling so that the center conductors of the coaxial linesneed not extend through the connector. The result is a ruggedized widefrequency band centerless coaxial connector having improved resistanceto high orders of vibration, temperature and shock. The device of theinstant invention simultaneously obviates the requirement of accuratecable alignment during coupling, thereby enabling considerable reductionof the usually required high order of manufacturing tolerances.

The principal or predominant mode of transmission of a coaxial cable isgenerally known `as the TEM mode and an electromagnetic wave of anyfrequency may be transmitted in this mode. `It is also possible on suchcoaxial transmission lines to generate higher order modes which do noteliminate the center conductor of the coaxial cable but do approach thecharacteristics of a circular wave guide as the diameter of the centerconductor approaches zero. Such higher order modes (TEM, TE21, T1531,TEM, TMll, etc.) have not been generally acceptable because of thecoupling difficulties encountered when using such modes and the furtherdisadvantage in that they require the use of complex mode suppressorfilters which cornplicate the design of standard transmission lines aswell as introduce inherent losses into the line. There is, however, amode known in circular wave guide theory as the TMm mode which is verysimilar to the TEM mode of the coaxial cable. The device of the instantinvention seizes upon this similarity of the TMm mode to eliminate thecenter conductor of the coaxial cable at the coupling point. Theresulting structure is considerably simpler than previously utilized incoupling devices, resolving itself into a mode transitional element ofvery wide operational frequency characteristics.

Thus, it will be `apparent that one object of the present invention isthe provision of a connector for coaxial cables which obviates the needfor center conductor contact fingers.

Another object is to provide a centerless coaxial connector which may berapidly utilized to connect two sections of coaxial cable togetherwithout the requirement of critical alignment, thus providing a quickdisconnect device.

A further object of the invention resides in the provision of acenterless coaxial cable which is extremely rugged and has a -highresistance to vibration, temperature and shock.

Still another object is to provide a centerless coaxial connectorincorporated as an end termination on the coaxial cable itself and whichdoes not have the center conductor of the coaxial cable extendingthrough said connecting end termination portion.

Yet yanother object of the present invention is the provision of acenterless coaxial cable connector which transforms the TEM mode of thecoaxial cable to the TMm mode of Ia circular wave guide at the point ofcoupling,

A still further object of the present invention is to provide a novelcoaxial cable embodying a connector having all of the aforementionedfeatures as an end termination integrally formed with the cable itself.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the sa-me becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is an axial section through a coaxial transmission line showing agraphical representation of the TEM mode of a coaxial cable;

FIG. 2 is an axial section through the centerless coaxial connector ofthe instant invention showing a graphical representation of the TMm modeof a circular wave guide;

FIG. 3 is a longitudinal section through a pair of mating coaxial cableseach embodying a centerless end termination in accordance with theinstant invention;

FIG. 4 is a longitudinal section through a pair of coaxial cablescoupled together by means of the centerless coaxial connecting expedientof the instant invention and wherein the two cables are held in buttjoint relation by means of a threaded retaining ring;

FIG. 5 is a longitudinal section of a pair of coaxial cables embodyingthe centerless coaxial connecting expedient of the instant invention andutilizing a simple choke connection to hold the cables together; and

FIG. 6 is an exploded view in perspective of the cable arrangement shownin FIG. 5.

Referring now to the drawings, wherein like reference characters denotelike or corresponding parts throughout the drawings, there is shown inFIG. 1 a graphical representation of the TEM mode of a coaxialtransmission line, the latter mode being the principal or predominantmode transmitted by such a line. It will be noted from FIG. 1 that theTEM mode requires the presence of both an outer cylindrical conductorand an inner coaxial conductor. The graphical representation of FIG. 2,on the other hand, shows that the TMm mode of a circular wave guide canbe sustained without the presence of a center conductor. The instantinvention takes advantage of the latter simplicity of structure capableof sustaining the TMUl mode to provide a unique coupling expedientbetween a pair of coaxial cables.

It will be observed that it is necessary to effect an efcient transitionfrom the TEM mode to the TMm mode. Referring now to FIG. 3 of thedrawings, the latter transition can easily be accomplished by the use ofa probe 12 placed longitudinally and coaxially with the center conductor14 of a coaxial cable, such as the cable 10, to be coupled. Such a probe12, preferably, is merely an extension of the center conductor 14itself, rather than consisting of a separate conducting element attachedto the latter center conductor. A coupling expedient is thereby providedfor inter-connecting adjacent ends of any two coaxial conductor ultrahigh frequency energy transmission lines, which may or may not beflexible.

Each transmission line, except for the end termination connectingportion, is of conventional construction comprising a exible solid wireinner conductor 14 or 15, a continuous insulator sleeve 16 or 17 of adielectric material which is suitably flexible and has sutiiciently highdielectric properties for the use desired, an outer conductor 18 or v19which is typically of braided copper to permit desired expansibility andflexibility, and an external electrically insulating covering (notshown) of a suitable material, such as rubber or saturated fabric. Theend termination centerless connecting portions 20 and 21 of the coaxialcables and 11 are filled with a second dielectric material 23, which mayor may not be the same material as the dielectric material utilized inthe remaining portions of the cables 10' and 11, as will be hereinafterdescribed in detail. The extension of the center conductors 14 and 15 ofcables 10 and 11 as probes 12 and l13 imbedded in the dielectricmaterial 23 of the connecting end terminationseifectively transforms theTEM mode of the cable into a TMol circular wave guide mode in the endtermination connecting portion. Ihe latter TMm mode is propagatedthrough the end termination connecting portion 20 of the first cable 110and then through the end termination connecting portion Z1 of a secondcable `.111, the centerless coaxial connector of which is butted againstthe connector of the first cable 10. Once having been propagated throughthe connecting portion 21 of the second cable 11 the TMm mode isreconverted in the second cable to the original TEM mode of the firstcoaxial cable.

FIG. 4 shows two coaxial cables embodying the coupling expedient of theinstant invention and held together by means of a cylindrical threadedretaining ring 24 which is used to clamp the two cable end portionconnectors 20 and 2.1 together in butt-joint relation. As is clearlyevident from FIG. 4, the accuracy of alignment of the two cable endportions 20 and 21 is not critical since no Contact nger probes arerequired in connecting the latter end portions together.

FIGS. 5 and 6 show a further embodiment of the invention utilizing thesame type of centerless coaxial connecting portions shown in FIGS. 3 and4 but using a choke joint connector 25 to hold the two coaxial cable endportions in butt-joint relation by means of a friction squeeze t. 'Ihelatter embodiment is especially suitable for quick disconnectapplications.

In dealing with centerless coaxial connectors of the circular wave guidetype, as contemplated by the instant invention, one objectionimmediately becomes apparent due to the cut-off frequencycharacteristics exhibited by such wave guide structures. That is, theproposed centerless coaxial connectors would be frequency sensitive.Another objection would appear to lie in the bulkiness of such astructure. For example, the diameter of an airilled circular X-band waveguide would have to be 1.38 inches if it were to be able to effectivelypropagate the TMm mode near the cut-olf frequency of y6.57 kmc. Thelatter figures may be readily calculated by utilizing the descriptivetheoretical information and equations set forth on pages 135 through 146of Electronic and Radio Engineering, by Frederick Emmons Terman, McGraw-Hill Book Company, Inc., New York, 1955, 4th edition, as follows:

where For the TMm mode in a circular waveguide,

Equation l therefore becomes Solving for r, the radius of the waveguide,

The diameter of the circular waveguide in inches is thus Ac=2.6lr= =4.57cm. (3)

or 1.38 inches, as stated.

From the above, it readily follows that in using the proper dielectricconstant `for the material `filling the waveguide structure, thediameter of the waveguide can be reduced to any convenient size, as forexample, the same size as the remaining portion 0f the coaxial cable forwhich it serves as an end termination. In such a case, the outer wall ofthe circular waveguide would merely be a continuation of the outercylindrical conductor for the remainder of the coaxial cable. In theabove example, let us say that the diameter of the circular waveguide isnot to exceed 0.280 inch or 0.710 cm. (r=0.355 cm.).

From Equation 4,

where Nc=cutol wavelength of the waveguide in the presence of the newdielectric filling material.

The following relation also exists:

where e=dielectric constant of the material iilling the circularwaveguide.

Solving Equation 7 for e,

)Je 2 4,6 8) Substituting the values of Nc and Ac yfor the instantproblem,

Hence the dielectric requirements for any particular diameter endtermination coaxial connector and for any desired cut-off frequency maybe readily determined by the above method of computation. Many materialshaving suitably high dielectric constants for such purposes haverecently become available in the form of ceramics. Continuing researchand subsequent development of improved dielectrics offering an evenWider range of available dielectric constants will further increase theapplicability of the centerless coaxial connector of the instantinvention.

The resulting devices shown in FIGS. 4, 5 and 6 exhibit extremely ruggedproperties including very high resistance to vibration, temperature andshock. Nor is great accuracy of alignment required in coupling any twocables embodying the instant invention since no troublesome contactfingers are involved. It is believed that the proposed centerlesscoaxial connector of the instant invention directly incorporated as anend termination into a coaxial cable, is an item that provides asubstantial advance in the coaxial connector art.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

Having thus described the invention, what is claimed is:

1. In a system for the transmission of radio frequency energy, a pair ofcoaxial cables, each of said cables having inner and outer coaxialelectrical conductors separated by a first and -a second dielectricmaterial, and an end structure on each of said cables including acircular cylindrical outer conductor extending a predetermined distancebeyond said first dielectric material `and filled with said seconddielectric material, the inner conductor of each of said coaxial cablesextending only partly into and sealed within the second dielectricmaterial of its respective cable end structure, the outer conductors ofsaid end structures being integral with the outer conductors of theirrespective coaxial cables, and means including -a member adjustablymounted on one end structure and snugly engaging the other end structurefor clamping the said end structures of each of said cables and thesecond dielectric material therein in butt-joint relation.

2. A centerless connecting end termination for a coaxial cable havinginner and outer coaxial electrical conductors separated by la firstdielectric composed of a flexible material and a second dielectriccomposed of a different material than the first dielectric, said endtermination comprising a circular cylindrical outer conductor extendingbeyond said first dielectric and filled with said second dielectricmaterial, the inner conductor of said coaxial cable extending onlypartly into the second dielectric material of said end termination andembedded therein, and the outer conductor of said end termination beingintegral with the outer conductor of said cable.

3. In a system for the transmission of radio frequency energy, a coaxialcable having inner and outer coaxial electrical conductors, a firstdielectric composed of a flexible material and electrically insulatingsaid conductors from each other, and an end structure consisting of anintegral extension of said outer electrical conductor of said coaxialcable filled with a second dielectric cornposed of a substantially solidmaterial, the inner conductor of said coaxial cable having a probethereon and embedded in said second dielectric material short of theterminal end of the extension on the outer conductor.

4. In a system for the transmission of radio frequency energy, aplurality of coaxial cables, each having outer and inner coaxialelectrical conductors separated by dielectric material, connected one toanother at their respective ends by means of an end structure on each ofsaid cables, said cable end structures each consisting of an outerelectrically conductive sleeve filled entirely with dielectric material,the dielectric material of the end structures differing from thedielectric material in the remaining portion of the coaxial cable andmeans on the end structure of each cable for maintaining the endstructure and the dielectric material therein in butt-joint relation onone end structure and a retaining ring threaded on the other endstructure in clamping engagement with the flange.

5. 'In a system for the transmission of radio frequency energy, firstmeans for conducting said radio frequency energy in the TEM mode, secondmeans for conducting said radio frequency energy in the TEM mode, saidfirst and second means including coaxial cables each having outer andinner coaxial electrical conductors and a dielectric of solid materialelectrically insulating said conductors from each other, said innerconductor terminating short of the terminal end of the outer conductorand having `a probe embedded in the dielectric material, and third meanscarried by the outer conductor of one cable, complementary means carriedby the outer conductor of the other cable in engagement with the meanson said one cable, for coupling and maintaining said rst and secondmeans together in abutting relationship, the predominant mode oftransmission of said third means being a mode other than the TEM mode.

6. In a system for the transmission of radio frequency energy, firstmeans for conducting said radio frequency energy in the TEM mode, secondmeans for conducting said radio frequency energy in the TEM mode, saidfirst and second means including a pair of coaxial cables having outerand inner coaxial electrical conductors and a dielectric of solidmaterial electrically insulting said conductors for each other, saidinner conductors terminating short of the terminal ends of outerconductors and having a probe imbedded in the dielectric material, andthird means inserted between said first and second means for couplingsaid first and second means together, said third means including -acircular waveguide structure having an outer cylindrical conductivesleeve on the terminal end of the conductor of one cable and overlappingthe terminal end of the outer conductor of the other cable and in spacedrelationship thereto and filled with the dielectri material, thepredominan-t mode of transmission of said third means being the TMmmode.

References Cited in the file of this patent UNITED STATES PATENTS2,527,146 Mumford Oct. 24, 1950' 2,683,251 Ramo July 6, 1954 2,746,018Sichak May 15, 1956 2,929,034 Doherty Mar. l5, 1960

2. A CENTERLESS CONNECTING END TERMINATION FOR A COAXIAL CABLE HAVINGINNER AND OUTER COAXIAL ELECTRICAL CONDUCTORS SEPARATED BY A FIRSTDIELECTRIC COMPOSED OF A FLEXIBLE MATERIAL AND A SECOND DIELECTRICCOMPOSED OF A DIFFERENT MATERIAL THAN THE FIRST DIELECTRIC, SAID ENDTERMINATION COMPRISING A CIRCULAR CYLINDRICAL OUTER CONDUCTOR EXTENDINGBEYOND SAID FIRST DIELECTRIC AND FILLED WITH SAID SECOND DIELECTRICMATERIAL, THE INNER CONDUCTOR OF SAID COAXIAL CABLE EXTENDING ONLYPARTLY INTO THE SECOND DIELECTRIC MATERIAL OF SAID END TERMINATION ANDEMBEDDED THEREIN, AND THE OUTER CONDUCTOR OF SAID END TERMINATION BEINGINTEGRAL WITH THE OUTER CONDUCTOR OF SAID CABLE.